It’s Not Easy Being Green

Following the lead of Neil DeGrasse Tyson on Twitter, I thought I’d write a quick note in honor of St. Patrick’s Day (at least as those of us in the United States tend to stereotype it) and the associated color green. I have taught astronomy quite a few times over the years (though I’m not technically an astronomer), and one weird fact that always boggles students minds is that although there are red stars, orange stars, yellow stars, white stars, and blue stars, there are no green stars (or purple stars, for that matter).

In fact, our Sun’s light peaks in the green part of the spectrum. The figure (click to enlarge) shows the energy output of stars with a variety of surface temperatures, known as thermal spectra. Our Sun has a surface temperature in between the yellow and green lines, so it peaks in the middle of the visible light part of the spectrum (roughly 400 to 700 nanometers (nm), or 0.000016 inches to 0.000028 inches for those who aren’t metrical)—right around the green part of the rainbow. But here’s the deal: if you look at that green line, you see that it may peak at green, it spreads out and covers most of the rainbow, so we actually would see that “green” star—our Sun*!—as white, which is a mixture of all the colors!

In addition, human are optimized for the light our Sun produces, so our receptor cells have peak performance right at the maximum solar output. (Other animals are able to see infrared and/or ultraviolet light, and have different levels of color perception.) Thus, “green” stars will never appear as a green color to human eyes simply because we are adapted to life under our Sun—any similar star or object with the same temperature range will also be perceived as white.

* As a side note, we often refer to our Sun as yellow, but that’s also a bit of a misnomer: if our Sun were actually yellow, white objects (which reflect back all the visible colors equally) would appear yellow as well.

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